1. Field of the Invention
The present invention relates to downhole tools used in oil and gas drilling operations, and more particularly, to telemetry and power-generating means for measurement-while-drilling (MWD) tools and processes used in such operations.
2. Background of the Related Art
The drilling of oil and gas wells typically involves the use of several different measurement and telemetry systems to provide data regarding the subsurface formation penetrated by a borehole, and data regarding the state of various drilling mechanics during the drilling process. In measurement-while-drilling (MWD) tools, data is acquired by sensors located in the drill string near the bit. This data is either stored in downhole memory or transmitted to the surface using a telemetry means, such as mud flow telemetry devices.
Both the downhole sensors and the telemetry means of the MWD tool require electrical power. Since it is not feasible to run a power supply cable from the surface through the drill string to the sensors or the telemetry means, electrical power must be obtained downhole. The state of the art MWD devices obtain such power downhole either from a battery pack or a turbine-based alternator. Examples of alternators used in downhole tools are shown in U.S. Pat. No. 5,517,464, assigned to the assignee of the present invention, and U.S. Pat. No. 5,793,625 assigned to Baker Hughes.
Turbine-based alternators employ rotors having impellers that are placed in the high-pressure flow of drilling fluid (“mudflow”) inside the drill string so that the impeller blades convert the hydraulic energy of the drilling fluid into rotation of the rotor. The rotors rotate at an angular velocity (speed) that provides enough energy to the MWD tools to power the telemetry means (e.g., a modulator) and sensors, and—in some cases—other tools in the drill string bottom-hole assembly (BHA).
In most conventional designs, the rotor of the turbine is coupled to a drive shaft which is coupled to an alternator, either directly or via a gear train that adapts the rotor's rotational speed for optimum operation of the turbine and alternator. The drive shaft is supported by bearings. Typically, the shaft, bearings, gear train, and alternator are all housed in a pressurized oil chamber in order to function in clean and well-lubricated conditions. Since the upstream portion of the drive shaft is rotating in drilling fluid, a rotary seal is required to isolate the drilling fluid from the oil in the pressurized chamber. The face of a typical rotary seal has to be lubricated by something other than the drilling fluid, since the drilling fluid contains erosive particles that will quickly ruin the rotary seal. This lubrication is achieved by ensuring a constant, low-volume oil leak from the pressurized chamber across the rotary seal. This leak also prevents the flowing drilling fluid from invading the oil chamber, which is desirable since the cleanliness of the oil promotes a long operating life for the gears, bearing, and electrical components inside the oil (i.e., drilling fluid particles would erode moving parts and damage the alternator components.) A well-known solution for achieving this controlled leakage of oil across the rotary seal is to employ a compensating piston that is biased by a spring having an appropriate spring constant within the pressurized oil chamber. For drilling efficiency, this piston is required to move a significant distance over time, which makes the spring and the chamber longer and bulkier than they might otherwise have to be. Accordingly, the compensating piston/spring assembly tends to make the downhole provision of power expensive. Furthermore, experience in the art has proven that a significant percentage of the failures and maintenance costs associated with downhole alternators are due to the extended length of the piston/spring oil reservoir pressure-compensation system. Still further, such a piston/spring configuration is, in many applications, an inefficient method of providing the necessary pressure rise needed to match the mud pressure. This is particularly true in tool configurations wherein the rotary seal is exposed to high hydrostatic-pressure drilling fluid (i.e., mud introduced at an upstream location), and the oil pressure must be raised to a magnitude several hundred pounds per square-inch (psi) above the local downstream mud pressure.
A need therefore exists for a pressure compensation that is not burdened by such shortcomings.